CHAPTER III 



HEREDITY AND VARIATION 



" The organic world as a whole is a perpetual flux of changing types." — 



Francis Galton. 



" Inheritance and variation are not two things, but two imperfect views 

 of a single process." — W. K. Brooks. 



" Variation and inheritance are, at present, one fundamental mystery 

 of the vital unit." — Karl Pearson. 



§ I. Persistence and Novelty. 



§ 2. The Tendency to Breed True. 



§ 3. Different Kinds of Organic Change. 



§ 4. Classification and Illustration of Variations. 



§ 5. Fluctuating Variations. 



§ 6. Discontinuous Variations. 



§ 7. De Vries on Fluctuations and Mutations. 



§ 8. Causes of Variation. 



§ I. Persistence and Novelty 



Close observers of the relation between successive generations in 

 mankind, or among plants and animals, are at one in record- 

 ing two distinct impressions, — on the one hand, of persistent 

 hereditary resemblance, on the other hand, of variability. 

 Oftenest we are first impressed by the remarkable homogeneity 

 which obtains from generation to generation, but as we get to 

 know the organisms better we become aware of individual 

 traits standing out against the background of general similarity. 

 Or it may be that, with the partiality of parents, our first 



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